The enteric protozoa Cryptosporidium parvum, Giardia lamblia and Entamoeba histolytica, and the enteric bacteria Shigella dysenteriae type 1 and Enterohemorrhagic E. coli are the highest priority enteric pathogens in Category B according to the NIAID Blue Ribbon Panel. An important reason is because they are difficult to accurately diagnose. This is due to the insensitivity of our best methods to detect the pathogen and, because all major Category B enteropathogens are capable of causing asymptomatic intestinal infection, the weak positive predictive value of a positive stool result in implying disease causation even when the pathogen is accurately detected. Therefore the first need is for highly sensitive tests for detecting Category B pathogens in stool, and Aim 1 will rigorously validate such tests that come on-line from Projects 1 and 2. This validation will occur in Bangladesh and Tanzania because of the hyperendemic rates of diarrheal illness due to Category B enteropathogens and will be expeditious due to ongoing, productive collaborations between Techlab, the University of Virginia, the International Centre for Diarrhoeal Diseases and Research in Bangladesh, and the Kilimanjaro Christian Medical Centre in Tanzania. The second need for Category B enteric diagnostics is for enhanced specificity in the setting of a positive test result. This is particularly problematic for the Category B protozoa because of their high rates of asymptomatic carriage and widespread environmental contamination. Aim 2, therefore, will correlate Giardia and Cryptosporidium genotype with clinical symptomatology through a case-control study. This will also occur in Bangladesh in Tanzania because of high local rates of asymptomatic infection from Category B protozoa, which permits a highly-powered analysis. Through the work of Project 3 we envision having sensitive diagnostics for the field and the public health laboratory, and an ability to streamline outbreak investigations by knowing which virulence genotypes to track and which asymptomatic, incidentally-detected infections to ignore.